haiku/docs/user/support/Unarchiver.dox

/*
 * Copyright 2010, Haiku, Inc. All Rights Reserved.
 * Distributed under the terms of the MIT License.
 *
 * Author:
 *		Alex Wilson, yourpalal2@gmail.com
 *
 * Corresponds to:
 *		/trunk/headers/os/support/Archivable.h rev 37751 
 *		/trunk/src/kits/support/Archivable.cpp rev 37751
 */


/*!
\class BUnarchiver
\ingroup support
\ingroup libbe
\brief A class that simplifies the unarchiving of complicated BArchivable
	hierarchies.

	The BUnarchiver class is a small class used to recover BArchivable objects
	that have been archived with the BArchiver class. It also provides ownership
	semantics, so that memory leaks can be avoided during the unarchival
	process. When retrieving an object (either via GetObject() or FindObject()),
	you can specify a BUnarchiver::ownership_policy. If you specify
	BUnarchiver::B_ASSUME_OWNERSHIP, you will become responsible for deleting
	the retrieved item. If you specify BUnarchiver::B_DONT_ASSUME_OWNERSHIP,
	you will not become responsible. You cannot take ownership of the same
	object twice. After the unarchival process finishes, any unclaimed objects,
	excluding the root object (the object being instantiated via
	instantiate_object() or BUnarchiver::InstantiateObject()), will be deleted.

	If you are updating a class that previously did not use the BArchiver and
	BUnarchiver helper classes, and want to maintain backwards compatibility
	with old archive, this can be done using the IsArchiveManaged() method. 

 	\Warning Calling methods on your BUnarchiver with a legacy archive (one that
		was not managed by a BArchiver during archival) will result in a
		call to debugger().
*/


/*!
	\fn BUnarchiver::BUnarchiver(const BMessage* archive)
	\brief Constructs a BUnarchiver object to manage \c archive.

	\note To guarantee that your AllUnarchived() method will be called during
		archival, you must create a BUnarchiver object in your archive
		constructor. It is necessary to do this even if you won't use the
		BUnarchiver object in your archive constructor.

	\warning Do not construct a BUnarchiver object without first calling
		BUnarchiver::PrepareArchive() on \c archive. It is only safe to build a
		BUnarchiver without this call in your AllUnarchived() implementation. 

	\see BUnarchiver::PrepareArchive()
*/


/*!
	\fn BUnarchiver::~BUnarchiver()
	\brief Destroys a BUnarchiver object. Calls this objects Finish() method,
		if it has not yet been called.
*/


/*!
	\fn status_t BUnarchiver::EnsureUnarchived(int32 token)
	\brief Ensure the object represented by \c token is unarchived and
		instantiated.
*/


/*!
	\fn status_t BUnarchiver::EnsureUnarchived(const char* name,
		int32 index = 0)
	\brief Ensure the object archived under \c name at \c index is unarchived
		and instantiated.
*/


/*!
	\fn bool BUnarchiver::IsInstantiated(int32 token)
	\brief Checks whether the object represented by \c token has been
		instantiated in this session.
*/


/*!
	\fn bool BUnarchiver::IsInstantiated(const char* name, int32 index = 0)
	\brief Checks whether the object archived under \c name at \c index has been
		instantiated in this session.
 */


/*!
	\fn template<class T> status_t BUnarchiver::GetObject(int32 token,
		ownership_policy owning, T*& object)

	\brief Recover an object by token that was archived by a BArchiver object.
	If the object has not yet been instantiated, and this request is not coming
	from an AllUnarchived() implementation, the object will be instantiated now.

	If the retrieved object is not of the type \c T, then this method will fail.
	If this method fails, you will not receive ownership of the object, no
	matter what you specified in \c owning.
	
	\tparam T The type of object you wish to find.

	\param token The token you got for this object from
		BArchiver::GetTokenForArchivable() during archival.
	\param owning Whether or not you wish to take ownership of the
		retrieved object.
	\param object Return parameter for the retrieved object of type \c T.

	\retval B_BAD_TYPE The object retrieved was not of type \c T.
*/


/*!
	\fn template<class T> status_t BUnarchiver::GetObject(int32 token,
		T*& object)

	\brief Recover and take ownership of an object represented by \c token.

	Equivalent to calling GetObject(token, BUnarchiver::B_ASSUME_OWNERSHIP,
		 object)
*/


/*!
	\fn template<class T> status_t BUnarchiver::FindObject(const char* name,
		int32 index, ownership_policy owning, T*& object)

	\brief Recover an object that had previously been archived using
		the BArchiver::AddArchivable() method. If the object has not yet been
		instantiated, and this request is not coming from an AllUnarchived()
		implementation, the object will be instantiated now.

	If the retrieved object is not of the type \c T, then this method will fail.
	If this method fails, you will not receive ownership of the object, no
	matter what you specified in \c owning.
	
	\tparam T The type of object you wish to find.

	\param name The name that was passed to BArchiver::AddArchivable() when
		adding this object.
	\param index The index of the object you wish to recover (0 based, like
		BMessage::FindData().
	\param owning Dictates whether or not you wish to take ownership of the
		retrieved object.
	\param object Return parameter for the retrieved object of type \c T.

	\retval B_BAD_TYPE The object retrieved was not of type \c T.
*/


/*!
	\fn template<class T> status_t BUnarchiver::FindObject(const char* name,
		int32 index, T*& object)

	\brief Recover and take ownership of an object that had previously been
		archived using the BArchiver::AddArchivable() method.
*/


/*!
	\fn template<class T> status_t BUnarchiver::FindObject(const char* name,
		ownership_policy owning, T*& object)

	\brief Recover an object at index 0 that had previously been archived using
		the BArchiver::AddArchivable() method.

	Equivalent to calling FindObject(name, 0, owning, object).
*/


/*!
	\fn template<class T> status_t BUnarchiver::FindObject(const char* name,
		T*& object)

	\brief Recover and take ownership of an object at index 0 that had
		previously been archived using the BArchiver::AddArchivable() method.

	Equivalent to calling FindObject(name, 0, BUnarchiver::B_ASSUME_OWNERSHIP,
		object).
*/


/*!
	\fn status_t BUnarchiver::Finish(status_t err = B_OK);
	\brief Report any unarchiving errors and possibly complete the archiving
		session.
	\return The first error reported in this unarchiving session, or B_OK.

	This method may finish an unarchiving session (triggering the call of all
	instantiated objects' AllUnarchived() methods) if the following conditions
	are true:
	\li No errors have been reported to this or any other BUnarchiver object
		within this session.
	\li This is the last remaining BUnarchiver that has not had its Finish()
		method invoked.
	If you call this method with an error code not equal to B_OK, then this
	unarchiving session has failed, instantiated objects will not have their
	AllUnarchived() methods called, and any subsequent calls to this method
	on any BUnarchiver objects in this session will return your error code.
	Furthermore, any objects that have been instantiated, but have not had
	their ownership assumed by another object will now be deleted (excluding
	the root object).
*/


/*!
	\fn	const BMessage*	BUnarchiver::ArchiveMessage() const
	\brief Returns the BMessage* used to construct this BUnarchiver. This is
		the archive that FindObject() uses.
*/


/*!
	\fn static bool BUnarchiver::IsArchiveManaged(const BMessage* archive)

	\brief Checks whether \c archive was managed by a BArchiver object.
	\retval true if \c archive was managed by a BArchiver object.
	\retval false otherwise.

	This method can be used to maintain archive backwards-compatibility for a
	class that has been updated to use the BArchiver class. If there is a
	possibility that you are may dealing with a legacy archive, you can use
	this method to find out before calling any methods on your BUnarchiver
	object.

	Here is an example of how you might use this method. Note that you
	must still call BUnarchiver::PrepareArchive(archive), either way.

	\code
MyArchivableClas::MyArchivableClass(BMessage* archive)
	:
	BArchivable(BUnarchiver::PrepareArchive(archive))
{
	BUnarchiver unarchiver(archive);

	if (BUnarchiver::IsArchiveManaged(archive)) {
		// ... calls to FindObject() or GetObject() here ...
	} else {
		// ... calls to BMessage::FindMessage() here ...
	}
}
	\endcode
*/


/*!
	\fn static BMessage* BUnarchiver::PrepareArchive(BMessage*& archive)
	\brief Prepares \c archive for use by a BUnarchiver.
	\param	archive The archive you wish to have prepared.
	\return The same BMessage as is passed in.

	This method must be called if you plan to use a BUnarchiver on an archive.
	It must be called once for each class an object inherits from that
	will use a BUnarchiver.

	\warning This method \bold must be called \bold before a call to the
		archive constructor of your parent class.

	Notice the use of this method in the example provided below.
	\code
MyArchivableClas::MyArchivableClas(BMessage* archive)
	:
	BArchivable(BUnarchiver::PrepareArchive(archive))
{
	// ...
}
	\endcode
*/


/*!
	\fn void BUnarchiver::AssumeOwnership(BArchivable* archivable)
	\brief Become the owner of \c archivable.

	After calling this method, you are responsible for the deletion
		of \c archivable.
*/


/*!
	\fn void BUnarchiver::RelinquishOwnership(BArchivable* archivable)
	\brief Relinquish ownership of \c archivable. If \c archivable remains
		unclaimed at the end of the unarchiving session, it will be deleted
		(unless it is the root object).
*/


/*!
	\fn template<class T> status_t BUnarchiver::InstantiateObject(
		BMessage* from, T*& object)
	\brief Attempt to instantiate an object of type \c T from BMessage* \c from.

	If the instantiated object is not of type \c T, then it will be deleted,
	and this method will return \c B_BAD_TYPE. This method is similar to
	the instantiate_object() function, but provides error reporting and 
	protection from memory leaks.
*/